1. Field of the Invention
The present invention relates to a high-resolution optical encoder that detects positions in a servo system.
2. Description of the Background Art
Encoders have been popularly used as displacement sensors in servo systems such as numerical control (NC) machine tools. Encoders detect rotation of motors, linear motion, and position, angle, velocity, and other displacement of a body in rotary motion. Encoders generally detect displacement information either magnetically or optically.
Optical encoders has a housing that houses a light source and a light receiving section which receives light from the light source, and an optical scale (i.e. a code disk) connected to the rotating body as a moving object. For example, an optical rotary encoder that detects rotation of a rotating body has a rotary disk (a code disk) connected to the rotating body. On the code disk, slit-form light transmitting section and light shielding section are periodically arranged.
FIG. 12 is a cross-sectional view of detection section of optical encoder. The optical encoder has code disk 120, light source 122, and light receiving section 123. Code disk 120 is located between light source 122 and light receiving section 123. Code disk 120 has a predetermined pattern 121 obtained by vapor-depositing Cr to a glass sheet, and selectively receives light flux 124 through pattern 121 to detect a position. This kind of code disk 120 has problems such as easy to break, heavy, and high cost.
As against such problems, code disks formed with plastics have been used (See Japanese Laid-open Patent Publication No. 11-23321). FIG. 13A shows a cross-sectional view of code disk 130 formed with plastics. Substrate 132 of code disk 130 has a recess, in which specified patterns that optically modulate an incident light in a specified direction are formed. FIG. 13B shows an enlarged cross-sectional view of substrate 132. Pattern 136 is formed on a bottom surface of recess 134 of substrate 132. Code disk 130 is rotated by rotary shaft P and detects rotation based on the incident light from a light source detected via pattern 136 of recess 134.
Because code disk 130 shown in FIG. 13A has single pattern 136 on the whole flat bottom surface of recess 134, that is, it only has a single track, code disk 130 can only detect an amount of relative displacement. With this configuration, the code disk is unable to determine an absolute position in the case that power supply of servo system is turned off or reset.